LEDs are becoming more widespread as light sources for general illumination purposes (e.g. as retro-fit light bulbs) but also for high-power applications such as automotive front lighting. A light-emitting diode (LED) comprises a semiconductor chip with an anode and a cathode. To include such an LED in an electronic circuit, it is generally provided in the form of a “package” ready for mounting, for example as a bottom-contacted chip with electrode pads—i.e. an anode pad and a cathode pad—arranged on the underside of the chip. These electrode pads can be bonded to appropriate conductors of a printed circuit board (PCB), previously etched from a conductive coating applied to a dielectric material layer.
An LED generates heat during operation, and the amount of heat is related to the power density of the LED. To avoid deterioration of the diode p-n junction owing to thermal damage, the heat must be drawn away from the LED. The trend is towards higher-power LEDs with smaller LED emission surface areas (smaller dies), and with correspondingly high power density (W/mm2). High-power LEDs of the type used in lighting applications can become extremely hot. Therefore, to draw heat away from a bottom-contacted LED, a heatsink is generally mounted on the opposite side of the PCB. The primary heat sink is therefore accessed through the PCB, and a lead frame is used to channel the heat from the LED into the PCB material. Heat from the anode and cathode pads is taken up by the conductive tracks to which they are bonded, passes through the dielectric layer, and is taken up or dissipated by the heatsink. The dielectric layer or thermal-electrical interface is problematic since it has a detrimental affect on heat transfer from the LED. This is because the dielectric is primarily an electric isolator such as a polymer, meaning that it is generally a poor thermal conductor, with a thermal conductivity that is typically significantly lower than the thermal conductivity of most metals. The dielectric layer of a PCB therefore effectively presents a thermal barrier in the thermal path away from the LED.
Another problem with the prior art approaches is that the conductive tracks on a conventional printed circuit board are very thin, usually only a few tens of micrometres in thickness, and therefore have only a limited thermal capacity. In one approach to dealing with this problem, the conductive tracks are not etched from a thin layer of copper material, but are instead formed from a relatively thick layer of conductive material (a few hundreds of micrometres in thickness) using a suitable material removal technique such as micro-milling. However, even if the conductive tracks are made somewhat thicker to increase their thermal capacity, this has been observed to be insufficient for dealing with the quantities of heat generated by a high-power LED during operation.
Therefore, it is an object of the invention to provide an improved way of dissipating heat from an LED during operation that overcomes the problems mentioned above.